Two-photon excitation of nuclei

An analysis is made of the two-photon absorption of radiation by nuclei subjected to two opposed $gamma$-ray fluxes. The absorption cross section has two maxima separated by the recoil energy. The higher maximum, of width GAMMA governed by the radiative damping, is associated with the recoilless absorption; the lower maximum, of width governed by the Doppler effect, describes the absorption in the presence of recoil. The low maximum makes it possible to attain low values of the parameter GAMMA/$omega$. (AIP)